Field of the Invention
The present invention relates to a light modulation element.
Description of Related Art
Currently, the amount of information used in optical communications is increasing steadily. In order to cope with an increase in the amount of information, countermeasures such as enhancing the signaling speed and increasing the number of channels by wavelength division coupling are progressing in optical communication networks such as the backbone, the metro, and the access. However, for this purpose, a system required for optical communication becomes complicated and there are problems such as large-sized and expensive equipment, increased power consumption, and the like.
Also in data centers that are increasing in recent years, it is necessary to cope with the increase in the amount of information. Conventionally, in the communication between computers in the data center, electric signals were mainly transmitted by metal cables. Meanwhile, from the demand for higher communication speed and reduction of power consumption, use of optical communication using optical fibers has been progressed in recent years. Furthermore, the introduction of optical communication is an issue for each level in the computer's board and in the CPU.
In recent years, optical devices using high refractive index semiconductor materials such as silicon, InP, and GaAs have attracted attention as a technology in the optical communication networks and further introduce optical communication to new fields.
When the above-mentioned materials are used, an optical waveguide having a strong optical confinement effect can be constructed and an optical device can be downsized. If the same function is employed, the optical device can be reduced in size and, if the same size is employed, more functions can be realized. In addition, it is possible to perform electrical control making full use of semiconductor material and it is possible to realize devices with variable characteristics such as optical modulators.
Furthermore, devices using silicon have many common elements with technologies/apparatuses related to semiconductor processes used in manufacturing semiconductor devices such as conventional CPU and memory such that low-cost devices can be expected at mass production. Integration with conventional semiconductor devices on the same substrate has the possibility of further speeding up equipment and lowering the power consumption by replacing part of the electric signals on metal wires so far with optical signals.
Under such circumstances, optical modulators are one of the main devices in optical communication in which electric signals are converted into optical signals. A Mach-Zehnder (MZ) type optical modulator is a representative constitution used as an optical modulator for an optical integrated device. The MZ type optical modulator is configured to branch light into two waveguides, subsequently couple the two waveguides by adding a phase difference, and modulate output light by interference.
In order to realize low frequency chirping, which is one of the features of the MZ type optical modulator, it is necessary to perform push-pull driving in which voltage is applied so as to generate phase modulation of opposite phase in two arms. In this case, the push-pull driving can be performed by applying a voltage such that the operation in each of the two arms is opposite in phase by differential driving using two high frequency signals.
On the other hand, when two high frequency signals are used, complication of electrode layout and an increase in footprint may be caused.
Japanese Unexamined Patent Application, First Publication No. 2006-65085 discloses that the structure of both arms is created so as to be symmetrical with respect to the center and a high frequency signal is output to both end portions while applying a bias to the central portion of the structure. It is disclosed that, with this structure, push-pull driving is performed by single driving using one high frequency signal without using differential driving.
In addition, L. Chen et al., “Chirp and dispersion tolerance of a single-drive push-pull silicon modulator at 28 Gb/s”, IEEE Photonics Technol. Lett. 24, 936-938 (2012) (hereinafter referred to as L. Chen et al.) discloses that push-pull driving with a single electrode is performed with a similar constitution using a silicon optical waveguide.
On the other hand, in the constitutions disclosed by Japanese Unexamined Patent Application, First Publication No. 2006-65085 and L. Chen et al., a voltage applied to a phase modulation section is divided to the respective arms and thus is made half of drive amplitude. Therefore, when it is attempted to attain the same modulation as the differential driving, the drive amplitude needs to be doubled, which may lead to an increase in power consumption.
S. J. Spector, et al., “CMOS-compatible dual-output silicon modulator for analog signal processing.,” Opt. Express 16, 11027-11031 (2008) (hereinafter referred to as S. J. Spector, et al.) discloses a constitution in which pnpn and a silicon waveguide are arranged from the periphery and a high frequency signal is output to the center thereof. In this constitution, since the drive amplitude is applied to each arm, modulation efficiency equivalent to that of the differential driving can be attained by single driving with voltage amplitude equivalent to one side of differential driving (p side or n side).
On the other hand, the constitution disclosed in S. J. Spector, et al. is a lumped constant type drive circuit and, because it is not of a traveling wave electrode type, high-speed modulation may be restricted. That is, since the speed of light is finite, the phase of the electric signal, particularly a high frequency component changes during the propagation through the waveguide and the modulation efficiency decreases, which leads to deterioration of a high frequency characteristic. If the length of the phase modulation section is shortened to avoid such disadvantage, the modulation efficiency may be lowered. Therefore, with the constitution disclosed in S. J. Spector, et al., it may be difficult to simultaneously achieve high-speed modulation and high modulation efficiency.
The present invention has been made in view of the above circumstances and provides a traveling wave electrode type light modulation element capable of high-speed modulation and capable of realizing single driving with high modulation efficiency.